Most of the medicaments for inhalation therapy have been dispensed as sprays from solutions either using a pressurized gas source for a separate solution in a spray device or as a self-contained gas source mixed with the medicament in a pressure dispensing container, frequently called an aerosol spray can.
Most commonly, the medicament is either a liquid, or dissolved in a solvent to be dispersed as a liquid, and if a volatile liquid is chosen, the solvent may evaporate to give an inhalable powder. Less commonly, the medicament is suspended as a powder in a propellant and dispersed as a dry aerosol directly. A major problem has been to secure effective dispersal in a desired particle size range. Patents and promotional literature are often unwarrantedly optimistic as to the effectiveness of dispersion.
For effective inhalation therapy where the drug is to be administered to the lower reaches of the lungs, it has been recognized that a particle size of the order of 0.5 microns to 10 microns is desired. Larger particles have a tendency to be deposited in the nasal or oral passages before the drug gets to the lung, and any effect on the lungs is by systemic absorption with the drug being circulated in the blood to the lungs. Sometimes the systemic effect of the drug on other organs is of dubious effectiveness or actually undesired, with what might be called topical application to the lungs being a preferred method of administration.